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Operand Consistency Attendant: An Associative Solution to the Data Field Consistency Problem

Publishing Venue

IBM

Related People

Brooks, AP: AUTHOR

Abstract

The operand consistency attendant (OCA) to be described is a hardware mechanism developed to prevent simultaneous, incompatible accesses to data fields. Without it, an unacceptable situation can arise in multiprocessing systems when one user is updating a field while another user is accessing it.

Country

United States

Language

English (United States)

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Operand Consistency Attendant: An Associative Solution to the Data Field
Consistency Problem

The operand consistency attendant (OCA) to be described is a hardware
mechanism developed to prevent simultaneous, incompatible accesses to data
fields. Without it, an unacceptable situation can arise in multiprocessing systems
when one user is updating a field while another user is accessing it.

In order to guarantee that competing processes do not simultaneously
access data except for compatible purposes, the OCA must compare the
operand addresses of each instruction being executed against every other
operand address in use. When a match is found, the intents of each user must
be reconciled. In this way, an update locks out all other accesses; several
inquirers may fetch at the same time, while executing updaters.

An operand address is available to the OCA at execution time. To determine
whether interference may occur over the entire operand, both starting and ending
addresses 1 and 3 are required. Assume that, at the machine language level,
the length of each operand is always known. It may be specified as part of the
instruction syntax, or it may be implied by the instruction decode circuitry. With
this assumption, the ending address 3 can also be readily generated. For
convenience, let ea be the ending address, and let o1 be the operand length-1.
As noted, ea = ba + o1. Adopting the subscript a to mean an active operand,
and subscript s to mean the subject operand, the relationships between these
operands is as follows. There is no overlap between the subject operand and an
active operand if: (1) ba(s) > ea(a) or (2) ea(a) < ba(a).

A simple OR of relations (1) and (2) will determine whether the operands
overlap. In parallel or multiprocessing machines, many data fields may be
accessed simultaneously. To compare all of the active fields with each new
subject operand efficiently requires an associative mechanism. In addition, in the
system described, many new subject operands are constantly being requested.
Most current systems allow several simultaneous storage accesses by utilizing
low-address-bit storage interleaving.

The time needed to detect less-than and greater-than relationships, even in
an associative manner, are necessarily dependent on the widths of the subjects.
On the other hand, the time needed to detect the equal relationship is
independent of the widths of the subjects. The following discusses how each
range may be approximated by a single address and a mask implying its range.
These addresses may then be compared for equality, exclusive of that portion of
each shaded by the greater mask. The latter method may be used to
approximate the OCA range overlap detection function.

Given the beginning address of a range and its length-1, the sum determines
the ending address of the range. In computing this sum, the highest order bit set
in the length-1 operand 2 may be determined, as can the highest order carry
generated. These...